1. Field Of The Invention
This invention relates to a gas detecting method and apparatus. More particularly, the invention relates to a method of selectively detecting a fuel gas having methane as a main component thereof and an incomplete combustion gas having carbon monoxide as a main component thereof, and to a gas detecting apparatus for use in executing this method. The invention employs a low heat capacity, hot-wire semiconductor type gas sensor having an oxide semiconductor formed mainly of tin oxide and acting as a sensitive section thereof.
2. Description Of The Related Art
Conventional semiconductor gas sensors for selectively detecting different types of gases as noted above are disclosed in Japanese Utility Model Publication No. 1993-32760 and Japanese Patent Publication Kokai No. 1992-147048.
The gas sensor disclosed in the former publication is intended to detect carbon monoxide as incomplete combustion gas, and methane, butane and the like as fuel gas. This sensor detects the incomplete combustion gas at a relatively low temperature in the order of 80.degree. C., and the fuel gas at a high temperature in the order of 400.degree. C. The sensor includes a sensitive section formed of an oxide semiconductor having tin oxide as a main component thereof. The sensitive section has a sensitizer such as palladium added thereto to increase its sensitivity. The fuel gas may be detected in a relatively short time (20 to 30 sec.), but a relatively long time of at least 90 sec. is required for detecting carbon monoxide.
The gas sensor disclosed in the latter publication is similar to that disclosed in the former publication in its object of detecting gases. The latter has relatively high temperature ranges of detection (the sensitive section becoming 300.degree. C. when detecting an incomplete combustion gas, and 500 to 600.degree. C. when detecting a fuel gas). The sensitive section of this sensor is formed of a metallic oxide semiconductor having tin oxide as a main component thereof, with a trace of platinum added thereto to adjust its sensitivity.
This sensitive section has a small outside diameter not exceeding 1 mm, and the sensor itself has a relatively low heat capacity, to realize a shortened response time.
However, the conventional sensors noted above have the following drawbacks.
The gas sensor disclosed in Japanese Utility Model Publication No. 1993-32760 has a peak of sensitivity to carbon monoxide at a temperature below 90.degree. C. This gas sensor has selectivity for a fuel gas only in a low temperature range of 40 to 80.degree. C.
In such a temperature range, therefore, some time is required for the sensor to adsorb carbon monoxide and reach equilibrium. The sensor has a poor response performance with a slow output response, taking 90 seconds to effect a reproducible detection of a CO concentration. Carbon monoxide is a highly dangerous gas, its permissible concentration (threshold limit value) being at 50 ppm. It is therefore desirable to detect leakage within a shorter time.
Where the oxide semiconductor has a precious metal such as palladium or platinum to act as a sensitizer, carbon monoxide which is flammable, usually, is partially burned at 100.degree. C. or above, instead of reaching the sensor interior. Consequently, the sensor inevitably has a low sensitivity to carbon monoxide.
With the gas sensor disclosed in Japanese Patent Publication Kokai No. 1992-147048, on the other hand, responsivity is secured by its low heat capacity (which actually is achieved by the small diameter sensitive section thereof) and the like. However, the sensitivity characteristics shown in FIG. 3 of this prior publication are data obtained from the respective gases at a high concentration of 4,000 ppm. The sensitivity to carbon monoxide remains low. As seen from
FIG. 4 of the publication, the selectivity for carbon monoxide is also low. That is, no selection can be made between carbon monoxide and methane around 500 ppm. It is impossible to determine with sufficient certainty whether detection is made of a gas resulting from incomplete combustion or a leakage of methane or the like used as fuel. Regarding concentration dependence, this gas sensor has a disadvantage of easily becoming saturated at a low concentration. Further, this gas sensor cannot be said to have sufficiently high sensitivity characteristics with respect to carbon monoxide. Since a fuel gas is detected at a high temperature range of 500 to 600.degree. C., particles constituting the oxide semiconductor tend to be sintered quickly, resulting in a short sensor life. To summarize the above facts, this sensor provides, at low temperatures, signals representing miscellaneous gases such as of hydrogen, carbon monoxide and alcohol, thereby to perform a function in the nature of air monitoring. The gas sensor has low measurement reproducibility and reliability as a carbon monoxide concentration measurement and alarm device for preventing incomplete combustion poisoning. The sensor cannot be said to be reliable for detecting and discriminating between gases generated through incomplete combustion and a fuel gas.